EP0851521A2 - Lithium ion secondary battery - Google Patents
Lithium ion secondary battery Download PDFInfo
- Publication number
- EP0851521A2 EP0851521A2 EP97122405A EP97122405A EP0851521A2 EP 0851521 A2 EP0851521 A2 EP 0851521A2 EP 97122405 A EP97122405 A EP 97122405A EP 97122405 A EP97122405 A EP 97122405A EP 0851521 A2 EP0851521 A2 EP 0851521A2
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- EP
- European Patent Office
- Prior art keywords
- active material
- positive
- separator
- negative electrode
- lithium ion
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M6/00—Primary cells; Manufacture thereof
- H01M6/04—Cells with aqueous electrolyte
- H01M6/06—Dry cells, i.e. cells wherein the electrolyte is rendered non-fluid
- H01M6/10—Dry cells, i.e. cells wherein the electrolyte is rendered non-fluid with wound or folded electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator supporting an electrolyte. More particularly, it relates to a battery structure having improved electrical connections between each of a positive electrode and a negative electrode and a separator, which needs no hard metallic case and allows a battery to take an arbitrary shape, such as a thin shape.
- a lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes.
- the lithium ion secondary batteries that have been put to practical use employ a positive electrode plate prepared by applying to an aluminum current collector a mixture comprising a powdered active material, such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin; a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
- a powdered active material such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin
- a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin
- Fig. 3 schematically illustrates the cross section of a conventional cylindrical lithium ion secondary battery disclosed, e.g., in JP-A-8-83608 (the term “JP-A” as used herein means an "unexamined published Japanese patent application”).
- reference numeral 1 indicates a battery case made of stainless steel, etc. which also serves as a negative electrode terminal
- numeral 2 an electrode body put into the battery case 1.
- the electrode body 2 is composed of a positive electrode 3, a separator 4, and a negative electrode 5 in a rolled-up form. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply pressure thereto from outside.
- Connecting means requiring no outer force that have been proposed to date include a structure in which a positive electrode and a negative electrode are adhered together with a liquid adhesive composition (electrolyte gel) as disclosed in U.S. Patent 5,460,904 and a structure in which a positive electrode and a negative electrode are formed by binding the respective active material with an electron conducting polymer, and the positive and negative electrodes are adhered together with a polymer electrolyte as disclosed in U.S. Patent 5,437,692.
- the structure in which electrodes are adhered with a liquid adhesive mixture is disadvantageous in that it makes the production process complicated, and sufficient adhesion is hardly obtained, and an improvement in mechanical battery strength is difficult to achieve.
- the structure in which electrodes are adhered to a polyelectrolyte has the following disadvantages.
- the polyelectrolyte layer should be thick so as to avoid a short-circuit between the electrodes, failing to make a sufficiently thin battery. Further, it is difficult for such a solid electrolyte layer to adhere to the active material, hardly achieving improvements in battery characteristics, such as a charge and discharge efficiency. The production steps involved are complicated, making the battery expensive.
- Efficiency of intercalation and deintercalation of lithium ions accompanying charge and discharge reactions of an active material is an important factor deciding the charge and discharge efficiency of a battery. Because, in a battery having an ordinary structure, mobility of lithium ions is equal throughout an electrolytic solution, intercalation and disintercalation of lithium ions occur deviationally in the vicinities of the electrode surfaces near the separator. As a result, the active material inside the electrode cannot be made effective use of, failing to obtain desired charge and discharge characteristics.
- the inventors of the present invention have conducted extensive study on a favorable method for adhering a separator to each of a positive electrode and a negative electrode.
- an object of the present invention is to provide a battery structure which makes it possible to adhere a separator to a positive electrode and a negative electrode firmly and intimately, thereby providing a lithium ion secondary battery which can have an increased energy density, a reduced thickness, and an arbitrary shape and yet exhibits excellent charge and discharge characteristics.
- a first aspect of the lithium ion secondary battery according to the present invention in the battery comprising:
- a second aspect of a lithium ion secondary battery of the present invention is a battery according to the first aspect wherein each of said positive and negative electrode active material layers is adhered to said separator with the same binder resin as used for adhering each of said positive active material particles and said negative active material particles to the respective current collectors.
- a third aspect of a lithium ion secondary battery of the present invention is a battery according to the first aspect wherein each of said positive and negative electrode active material layers is adhered to said separator with a second binder resin different from the binder resin used for adhering said positive active material particles and said negative active material particles to the respective current collectors.
- a fourth aspect of a lithium ion secondary battery of the present invention is a battery according to the first aspect wherein the coating ratio of said binder resin on said active material particles is higher in the side of said separator than in the side of said current collector.
- a fifth aspect of a lithium ion secondary battery of the present invention is a battery according to the first aspect wherein the density of the active material particles in each of the positive and negative electrode material layers in the side of the separator is lower than that of in the side of each of the positive and negative current collector.
- a sixth aspect of a lithium ion secondary battery of the present invention is a battery according to the first aspect wherein the density of the binder resin in each of the positive and negative electrode material layers in the side of the separator is higher than that of in the side of each of the positive and negative current collector.
- a seventh aspect of a lithium ion secondary battery of the present invention is a battery comprising:
- An eighth aspect of a lithium ion secondary battery of the present invention is a battery according to the seventh aspect wherein the density of the active material particles in each of the positive and negative electrode material layers in the side of the separator is lower than that of in the side of each of the positive and negative current collector.
- a ninth aspect of a lithium ion secondary battery of the present invention is a battery according to the seventh aspect wherein the density of the binder resin in each of the positive and negative electrode material layers in the side of the separator is higher than that of in the side of each of the positive and negative current collector.
- a tenth aspect of a lithium ion secondary battery of the present invention is a battery according to the seventh aspect wherein the active material particles have a particle size in a range of 0.3 to 20 ⁇ m.
- An eleventh aspect of a lithium ion secondary battery of the present invention is a battery according to the seventh aspect wherein the active material particles have a particle size in a range of 1 to 5 ⁇ m.
- Fig. 1 is a schematic cross section of the main part of a lithium ion secondary battery according to one embodiment of the present invention.
- Fig. 2 is a schematic cross section of a lithium ion secondary battery according to the embodiment of Fig. 1.
- Fig. 3 is a schematic cross section of an embodiment of conventional lithium ion secondary batteries.
- Fig. 1 is a schematic cross section of the main part (an electrode laminate) of a lithium ion secondary battery according to one embodiment of the present invention.
- Fig. 2 is a schematic cross section of a lithium ion secondary battery according to the embodiment of Fig. 1.
- numeral 3 indicates a positive electrode in which active material particles 7a are adhered to a current collector 6 by means of a binder resin 11.
- Numeral 7 is a positive electrode active material layer comprising active material particles 7a bound by a binder resin 11.
- Numeral 5 is a negative electrode in which active material particles 9a are adhered to a current collector 10 by means of a binder resin 11.
- Numeral 9 is a negative electrode active material layer comprising active material particles 9a bound by a binder resin 11.
- Numeral 4 indicates a separator that is interposed between the positive electrode 3 and the negative electrode 5 and is adhered to the positive and negative active material layers 7 and 9 with the binder resin 11.
- Numeral 12 indicates voids formed in the positive and negative active material layers 7 and 9 and the separator 4, where an electrolytic solution containing lithium ions is held.
- Numeral 30 is an aluminum laminate film which encloses and seals the electrode laminate shown in Fig. 1 after injection of the electrolytic solution.
- the lithium ion secondary battery having the above-described structure can be produced by, for embodiment, as follows.
- An active material paste prepared by dispersing positive electrode active material particles 7a and a binder resin 11 in a solvent is applied to a positive electrode current collector 6 by roll coating and dried to prepare a positive electrode 3.
- a negative electrode 6 is prepared in the same manner.
- the binder resin 11 is applied to the both sides of a separator 4 as an adhesive, and the positive electrode 3 and the negative electrode 6 are each adhered thereto.
- the adhesive is dried by hot roll pressing to prepare an electrode laminate.
- the whole electrode laminate is impregnated with an electrolytic solution by dipping and packed in an aluminum laminate film (resin coated aluminum film) 30.
- the film 30 is closed by heat sealing to obtain a lithium ion secondary battery.
- the adhesive strength between the separator 4 and each of the positive and negative electrode active material layers 7 and 9 is not lower than that between the positive electrode current collector 6 and the positive electrode active material layer 7 and that between the negative electrode current collector 10 and the negative electrode active material layer 9. Further, the coating ratio of the binder resin 11 on the active material particles 7a and 9a is higher in the side of the separator 4 than in the side of the current collector 6 and 10, respectively.
- the active materials and their respective collectors in the electrodes 3 and 5 are adhered together with the binder resin 11 to retain the shape of the electrodes as in conventional lithium ion secondary batteries. Since the positive and negative electrodes 3 and 5 (i.e., the positive and negative electrode active material layers 7 and 9) are similarly adhered to the separator 4 with the same binder resin 11, electrical connections between each of the active material layers 7 and 9 and the separator 4 can be maintained without applying an outer force. That is, a hard case for retaining a battery structure is no more necessary, which makes it feasible to reduce the weight and thickness of a battery and to design the shape of a battery freely.
- the separator 4 is adhered to each of the positive and negative active material layers 7 and 9 with such adhesive strength that is equal to or above the adhesive strength between the active material and its current collector in the inside of each electrode, i.e., between the positive electrode collector 6 and the positive electrode active material layer 7 and between the negative electrode current collector 10 and the negative electrode active material layer 9.
- adhesive strength that is equal to or above the adhesive strength between the active material and its current collector in the inside of each electrode, i.e., between the positive electrode collector 6 and the positive electrode active material layer 7 and between the negative electrode current collector 10 and the negative electrode active material layer 9.
- each electrode and the separator In order to further enhance the adhesion between each electrode and the separator thereby to make the above effect more outstanding, it is preferable to form a thin binder resin layer between each electrode and the separator.
- the binder resin serving as an adhesive in the positive and negative electrode active material layers 7 and 9 is more abundant in the side (on the surface) of the separator 4. That is, the active material particles 7a and 9a which are positioned near the separator 4 are coated with the binder resin 11 more than those positioned near the respective collectors 6 and 10. While intercalation and disintercalation of lithium ions are usually supposed to occur preferentially in the vicinities of the separator over the inside of the positive and negative active material layers 7 and 9, the difference in coating ratio in the thickness direction of the electrode layer relaxes the difference in speed of intercalation and disintercalation of lithium ions between the separator 4 side and the inside of the positive and negative electrode active material layers 7 and 9. As a result, the active material inside the electrode can be utilized effectively to improve the charge and discharge efficiency. It follows that the charge and discharge characteristics as a battery can be improved.
- the active material which can be used in the positive electrode includes complex oxides of lithium and a transition metal, such as cobalt, nickel or manganese; chalcogen compounds containing lithium or complex compounds thereof; and these complex oxides, Li-containing chalcogen compounds or complex compounds thereof that contain various dopant elements. While any substance capable of intercalating and disintercalating lithium ions, which take the main part of the battery behavior, can be used as a negative electrode active material, preferred active materials for use in the negative electrode include carbonaceous compounds, such as graphitizing carbon, non-graphitizing carbon, polyacene, and polyacetylene; and aromatic hydrocarbon compounds having an acene structure, such as pyrene and perylene.
- the active materials are used in a powdered state. Particles having a particle size of 0.3 to 20 ⁇ m can be used. A preferred particle size is 1 to 5 ⁇ m. In the case that the particle size is too small, the ratio of the surface area of the active material particles coated with the binder resin becomes too high to perform lithium ion intercalation and disintercalation efficiently, resulting in reduction of battery characteristics. In the case that the particle size is too large, it is not easy to form the active material paste into a thin film, the packing density is reduced, and the surface of the electrode plates formed has large unevenness, resulting in a failure of sufficient adhesion to the separator.
- the binder resin which can be used in binding the active material and forming into an electrode plate is not particularly limited as long as it is insoluble in an electrolytic solution and undergoes or induces no electrochemical reaction in the electrode laminate.
- useful binder resins are homo- or copolymers of vinylidene fluoride, ethylene fluoride, acrylonitrile, and ethylene oxide, and ethylene propylenediamine rubber.
- Any metal stable within a battery can be used as a current collector.
- Aluminum is preferred for a positive electrode, and copper is preferred for a negative electrode.
- the current collector can be foil, net, expanded metal, etc. Those presenting a large surface area, such as net and expanded metal, are preferred from the standpoint of ease of holding an electrolytic solution after adhesion.
- any separator which is electron-insulating and has sufficient strength such as porous film, net, and nonwoven fabric, can be used.
- a separator made of a fluorine resin there are some cases in which the adhesive strength should be increased by surface treatment, such as plasma treatment.
- surface treatment such as plasma treatment.
- polyethylene or polypropylene is a preferred material for the separator for their adhesiveness and safety.
- the solvent and the electrolyte which provide an electrolytic solution serving as an ion conductor can be any of nonaqueous solvents and any of lithium-containing electrolyte salts that have been employed in conventional batteries.
- useful solvents include ethers, such as dimethoxyethane, diethoxyethane, diethyl ether, and dimethyl ether; esters, such as propylene carbonate, ethylene carbonate, diethyl carbonate, and dimethyl carbonate; and mixed solvents consisting of two members selected from the ether solvents or the ester solvents or mixed solvents consisting of one member selected from the former group and one member selected from the latter group.
- useful electrolytes are LiPF 6 , LiAsF 6 , LiClO 4 , LiBF 4 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , and LiC(CF 3 SO 2 ) 3 .
- the adhesive resin which can be used for adhering a current collector and an electrode can be selected from those insoluble in an electrolytic solution and undergoes or induces no electrochemical reaction in the electrode laminate similarly to the adhesive resins used for adhering an electrode and a separator.
- useful adhesive resins are crosslinked methacrylic or acrylic resins and homo- or copolymers of ethylene, styrene, vinylidene fluoride, etc.
- a positive electrode active material paste Eighty-seven (87) parts of LiCoO 2 , 8 parts of graphite powder (KS-6, produced by Lonza Ltd ), and, as a binder resin, 5 parts of polyvinylidene fluoride (hereinafter abbreviated as PVDF) were dispersed in N-methylpyrrolidone (hereinafter abbreviated as NMP) to prepare a positive electrode active material paste.
- NMP N-methylpyrrolidone
- the resulting battery stably retained its shape without imposing pressure externally, keeping an electrical connection between electrodes.
- the active material layers were found remaining on the separator. This proves that the interlaminar adhesive strength between the separator and the active material layer is greater than the interlaminar adhesive strength between the current collector and the active material layer. This seems to be because the binder resin as an adhesive was present more on the separator side than on the current collector side. In this manner, because fracture of the electrode(s) occurs preferentially over interlaminar delamination between the active material layer(s) and the separator, the battery can maintain safety.
- the energy density per unit weight was 120 Wh/kg owing to effective utilization of the active materials in the inside of the active material layers. Even after 200 charge and discharge cycles at a current of C/2, the charge capacity was as high as 75% of the initial level. This is believed to be because the binder resin was present more on the separator side than on the current collector side. In other words, the active material particles positioned nearer to the separator were coated with the binder resin to a larger extent than those positioned nearer to the current collector, so that the difference in speed of intercalation and disintercalation of lithium ions between the separator side and the inside of the electrode active material layers might be reduced by offsetting and, as a result, the active materials inside the electrodes were made effective use of.
- a battery was produced in the same manner as in Embodiment 1, except that the positive and negative electrode active material layers both had a thickness of about 200 ⁇ m. Similarly to Embodiment 1, the resulting battery stably retained its shape without imposing pressure externally, keeping an electrical connection between electrodes. When, after the assembly, the aluminum laminate film was removed, and the electrodes were stripped off the separator, the active material layers were found remaining on the separator, proving that the interlaminar adhesive strength between the separator and the active material layer is greater than the interlaminar adhesive strength between the current collector and the active material layer.
- the energy density per unit weight was 103 Wh/kg and, even after 200 charge and discharge cycles at a current of C/2, the charge capacity was as high as 60% of the initial level. It was thus proved that a lithium ion secondary battery having reduced weight and thickness, an arbitrary shape, and excellent charge and discharge characteristics could be produced similarly to Embodiment 1.
- a battery was produced in the same manner as in Embodiment 1, except that a NMP solution containing 12% by weight of PVDF was used for adhesion of the positive and negative electrode active material layers to the separator. Similarly to Embodiment 1, the resulting battery stably retained its shape without imposing pressure externally, keeping an electrical connection between electrodes. When, after the assembly, the aluminum laminate film was removed, and the electrodes were peeled off the separator, the active material layers were found remaining on the separator, indicating that the interlaminar adhesive strength between the separator and the active material layer was greater than the interlaminar adhesive strength between the current collector and the active material layer. This is considered to be because a PVDF solution having a high concentration formed a thin PVDF layer between the separator and the electrode to enhance the interlaminar adhesive strength, maintaining an electrical connection stably.
- the energy density per unit weight was 90 Wh/kg and, even after 200 charge and discharge cycles at a current of C/2, the charge capacity was as high as 80% of the initial level.
- the charge capacity was as high as 80% of the initial level.
- a toluene solution containing 5% by weight of polystyrene was uniformly applied to a side of a nitrocellulose porous film (pore size: 0.8 ⁇ m) as a separator, and the positive electrode was stuck thereto uniformly. On the other side of the separator was uniformly applied the same toluene solution of polystyrene, and the negative electrode was stuck thereto.
- an electrolytic solution of lithium hexafluorophosphate (electrolyte) in a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane was injected into the electrode laminate, and the impregnated laminate was heat-sealed in an aluminum laminate film pack to complete a lithium ion secondary battery.
- the resulting battery stably retained its shape without imposing pressure externally, keeping an electrical connection between electrodes.
- the aluminum laminate film was removed, and the electrodes were peeled off the separator, the active material layers were found remaining on the separator, indicating that the interlaminar adhesive strength between the separator and the active material layer was greater than the adhesive strength in the inside of the electrode and the interlaminar adhesive strength between the current collector and the active material layer.
- the energy density per unit weight was about 90 Wh/kg, and even after 100 charge and discharge cycles at a current of C/10, about 60% of the initial charge capacity was retained.
- Embodiment 1 it was proved that a lithium ion secondary battery having reduced weight and thickness, an arbitrary shape, and excellent charge and discharge characteristics could be produced.
- a positive electrode and a negative electrode were prepared in the same manner as in Embodiment 1.
- the active material layers were found remaining on the separator, proving that the interlaminar adhesive strength between the separator and the active material layer was greater than the interlaminar adhesive strength between the current collector and the active material layer.
- the energy density per unit weight was about 103 Wh/kg, and even after 100 charge and discharge cycles at a current of C/2, the charge capacity was as high as about 80% of the initial level.
- the aluminum foil coated with the positive electrode active material and the copper foil coated with the negative electrode active material were pressed, while wet, onto each side a separator (Cellguard #2400, produced by Hoechst Celanese Plastics Ltd.) and dried to prepare an electrode laminate having the separator as an interlayer.
- a separator Cellguard #2400, produced by Hoechst Celanese Plastics Ltd.
- an electrolytic solution of lithium hexafluorophosphate (electrolyte) in a mixed solvent of ethylene carbonate and 1,2-dimethoxyethane was injected into the electrode laminate, and the impregnated laminate was heat-sealed in an aluminum laminate film pack to complete a lithium ion secondary battery.
- the resulting battery stably retained its shape without imposing pressure externally, keeping an electrical connection between electrodes.
- the active material was found remaining on the separator only sparsely. This indicates that the interlaminar adhesive strength between the separator and the active material layer is very lower than to that between the current collector and the active material layer.
- the energy density per unit weight was 70 Wh/kg. After 200 charge and discharge cycles at a current of C/2, the charge capacity was as low as 40% of the initial level.
- the resulting battery was considerably inferior in battery characteristics to those produced in the foregoing Embodiments, which verifies that adhering of positive and negative electrodes to a separator with an adhesive results in improvements in battery characteristics.
- the distribution of an adhesive and a binder resin plays an important role in improving battery characteristics.
- the first structure of the lithium ion secondary battery according to the present invention has a positive electrode comprising a current collector having adhered thereto positive electrode active material particles with a binder resin, a negative electrode comprising a current collector having adhered thereto negative electrode active material particles with a binder resin, a separator which is interposed between the positive electrode and the negative electrode and is adhered to each of positive and negative active material layers, and a lithium ion-containing electrolytic solution which is held in voids of the positive and negative electrode active material layers and the separator, in which the adhesive strength between the separator and each of the positive and negative electrode active material layers is not lower than that between the positive electrode current collector and the positive electrode active material layer and that between the negative electrode current collector and the negative electrode active material layer.
- a hard case for retaining a battery structure is no more necessary, which makes if feasible to reduce the weight and thickness of a battery and to design the shape of a battery freely. Additionally, since the charge and discharge efficiency can be improved, there is provided a lithium ion secondary battery having excellent charge and discharge characteristics and high safety.
- the second structure of the lithium ion secondary battery according to the present invention has the first structure, in which each of the positive and negative electrode active material layers is adhered to the separator with the same binder resin used for adhering the positive electrode active material particles and the negative electrode active material particles to the respective current collectors. According to this structure, the battery has improved reliability.
- the third structure of the lithium ion secondary battery according to the present invention has the first or second structure, in which the coating ratio of the binder resin on the active material particles is higher in the side of the separator than in the side of the current collector. According to this structure, the difference in speed of intercalation and disintercalation of lithium ions between the active material positioned near to the separator and that positioned near to the current collector can be relaxed so that the active material inside the electrode can be utilized effectively to bring about improved charge and discharge efficiency.
Abstract
Positive and negative active material particles 7a and 9a are
adhered to the respective current collectors 6 and 8 by means of
a binder resin 11 to prepare positive and negative electrodes 3
and 5. The positive and negative electrode active material layers
7 and 9 are adhered to a separator 4 with the binder resin 11 so
that the interlaminar strength between each active material layer
7, 9 and the separator 4 may be not lower than that between the
active material layer 7, 9 and the respective current collector
10, 9. A lithium ion-containing electrolytic solution is held
in voids 12 made in the active material layers 7, 9 and the
separator 4 to complete an electrical connection between the
electrodes.
Description
This invention relates to a lithium ion secondary battery
comprising a positive electrode and a negative electrode facing
each other via a separator supporting an electrolyte. More
particularly, it relates to a battery structure having improved
electrical connections between each of a positive electrode and
a negative electrode and a separator, which needs no hard metallic
case and allows a battery to take an arbitrary shape, such as a
thin shape.
There has been an eager demand for reduction in size and
weight of portable electronic equipment, and the realization
relies heavily on improvement of battery performance. To meet
the demand, development and improvement of a variety of batteries
have been proceeding. Battery characteristics expected to be
improved include increases in voltage, energy density, resistance
to high load, freedom of shape, and safety. Out of available
batteries, lithium ion batteries are secondary batteries that
have achieved the most high voltage, the most high energy density,
and the most excellent resistance to high load and will undergo
successive improvements.
A lithium ion secondary battery mainly comprises a
positive electrode, a negative electrode, and an ion conducting
layer interposed between the electrodes. The lithium ion
secondary batteries that have been put to practical use employ
a positive electrode plate prepared by applying to an aluminum
current collector a mixture comprising a powdered active material,
such as a lithium-cobalt complex oxide, a powdered electron
conductor, and a binder resin; a negative electrode plate prepared
by applying to a copper current collector a mixture of a powdered
carbonaceous active material and a binder resin; and an ion
conducting layer made of a porous film of polyethylene,
polypropylene, etc. filled with a nonaqueous solvent containing
lithium ions.
Fig. 3 schematically illustrates the cross section of a
conventional cylindrical lithium ion secondary battery disclosed,
e.g., in JP-A-8-83608 (the term "JP-A" as used herein means an
"unexamined published Japanese patent application"). In Fig. 3
reference numeral 1 indicates a battery case made of stainless
steel, etc. which also serves as a negative electrode terminal,
and numeral 2 an electrode body put into the battery case 1. The
electrode body 2 is composed of a positive electrode 3, a separator
4, and a negative electrode 5 in a rolled-up form. In order for
the electrode body 2 to maintain electrical connections among the
positive electrode 3, the separator 4, and the negative electrode
5, it is necessary to apply pressure thereto from outside. This
has been achieved by putting the electrode body 2 into the rigid
case 1 which is to apply pressure thereby keeping the planar
contacts among the above three elements. In the case of
rectangular batteries, an external pressing force is imposed to
a bundle of strips of an electrode body by, for embodiment, putting
the bundle in a rectangular metal case.
That is, a contact between a positive electrode and a
negative electrode with a separator therebetween in commercially
available lithium ion secondary batteries has been made by using
a hard case made of metal, etc. Without such a case, the electrode
body would be separated, and the battery characteristics would
be deteriorated due to a faulty electrical connection. However,
because the case occupies a large proportion in the weight and
volume of a battery, it contributes to not only reduction in energy
density of the battery but limitation of the battery shape, making
it difficult to make a battery of arbitrary shape.
Under such circumstances, development of lithium ion
secondary batteries which do not require a hard case has been
proceeding, aiming at reductions in weight and size (thickness).
The key to development of batteries requiring no hard case is
how to maintain an electrical connection between each of a
positive electrode and a negative electrode and an ion conducting
layer (i.e., separator) interposed therebetween without adding
an outer force.
Connecting means requiring no outer force that have been
proposed to date include a structure in which a positive electrode
and a negative electrode are adhered together with a liquid
adhesive composition (electrolyte gel) as disclosed in U.S.
Patent 5,460,904 and a structure in which a positive electrode
and a negative electrode are formed by binding the respective
active material with an electron conducting polymer, and the
positive and negative electrodes are adhered together with a
polymer electrolyte as disclosed in U.S. Patent 5,437,692.
Problem to be Solved by the Invention:
As mentioned above, conventional lithium ion secondary
batteries having the above-mentioned structure in which a hard
case is used for ensuring intimate contacts and electrical
connections between a positive electrode and a negative electrode
and a separator have the problem that the case which does not
participate in electricity generation has a large proportion in
the total volume or weight of a battery, which is disadvantageous
for production of batteries having a high energy density.
On the other hand, the structure in which electrodes are
adhered with a liquid adhesive mixture is disadvantageous in that
it makes the production process complicated, and sufficient
adhesion is hardly obtained, and an improvement in mechanical
battery strength is difficult to achieve.
The structure in which electrodes are adhered to a
polyelectrolyte has the following disadvantages. The
polyelectrolyte layer should be thick so as to avoid a short-circuit
between the electrodes, failing to make a sufficiently
thin battery. Further, it is difficult for such a solid
electrolyte layer to adhere to the active material, hardly
achieving improvements in battery characteristics, such as a
charge and discharge efficiency. The production steps involved
are complicated, making the battery expensive.
Efficiency of intercalation and deintercalation of
lithium ions accompanying charge and discharge reactions of an
active material is an important factor deciding the charge and
discharge efficiency of a battery. Because, in a battery having
an ordinary structure, mobility of lithium ions is equal
throughout an electrolytic solution, intercalation and
disintercalation of lithium ions occur deviationally in the
vicinities of the electrode surfaces near the separator. As a
result, the active material inside the electrode cannot be made
effective use of, failing to obtain desired charge and discharge
characteristics.
Hence, realization of a practically useful thin type
lithium ion battery requires development of a battery structure
that easily secures safety and strength as a battery and achieves
satisfactory battery characteristics such as charge and discharge
characteristics. In other words, it is required to develop a
structure having a separator between electrodes so as to secure
safety in which the separator is adhered to each electrode with
sufficient strength so that satisfactory battery characteristics
may be obtained.
In order to solve these problems, the inventors of the
present invention have conducted extensive study on a favorable
method for adhering a separator to each of a positive electrode
and a negative electrode.
Accordingly, an object of the present invention is to
provide a battery structure which makes it possible to adhere a
separator to a positive electrode and a negative electrode firmly
and intimately, thereby providing a lithium ion secondary battery
which can have an increased energy density, a reduced thickness,
and an arbitrary shape and yet exhibits excellent charge and
discharge characteristics.
A first aspect of the lithium ion secondary battery
according to the present invention in the battery comprising:
wherein the adhesive strength between said separator and each of said positive and negative electrode active material layers is not lower than that between said positive electrode current collector and said positive electrode active material layer and that between said negative electrode current collector and said negative electrode active material layer.
A second aspect of a lithium ion secondary battery of the
present invention is a battery according to the first aspect
wherein each of said positive and negative electrode active
material layers is adhered to said separator with the same binder
resin as used for adhering each of said positive active material
particles and said negative active material particles to the
respective current collectors.
A third aspect of a lithium ion secondary battery of the
present invention is a battery according to the first aspect
wherein each of said positive and negative electrode active
material layers is adhered to said separator with a second binder
resin different from the binder resin used for adhering said
positive active material particles and said negative active
material particles to the respective current collectors.
A fourth aspect of a lithium ion secondary battery of the
present invention is a battery according to the first aspect
wherein the coating ratio of said binder resin on said active
material particles is higher in the side of said separator than
in the side of said current collector.
A fifth aspect of a lithium ion secondary battery of the
present invention is a battery according to the first aspect
wherein the density of the active material particles in each of
the positive and negative electrode material layers in the side
of the separator is lower than that of in the side of each of
the positive and negative current collector.
A sixth aspect of a lithium ion secondary battery of the
present invention is a battery according to the first aspect
wherein the density of the binder resin in each of the positive
and negative electrode material layers in the side of the
separator is higher than that of in the side of each of the
positive and negative current collector.
A seventh aspect of a lithium ion secondary battery of
the present invention is a battery comprising:
wherein the adhesive strength between said separator and each of said positive and negative electrode active material layers is not lower than that between said particles in each of said positive electrode active material layer and said negative electrode active material layer respectively.
An eighth aspect of a lithium ion secondary battery of
the present invention is a battery according to the seventh aspect
wherein the density of the active material particles in each of
the positive and negative electrode material layers in the side
of the separator is lower than that of in the side of each of
the positive and negative current collector.
A ninth aspect of a lithium ion secondary battery of the
present invention is a battery according to the seventh aspect
wherein the density of the binder resin in each of the positive
and negative electrode material layers in the side of the
separator is higher than that of in the side of each of the
positive and negative current collector.
A tenth aspect of a lithium ion secondary battery of the
present invention is a battery according to the seventh aspect
wherein the active material particles have a particle size in a
range of 0.3 to 20 µm.
An eleventh aspect of a lithium ion secondary battery of
the present invention is a battery according to the seventh aspect
wherein the active material particles have a particle size in
a range of 1 to 5 µm.
Fig. 1 is a schematic cross section of the main part of
a lithium ion secondary battery according to one embodiment of
the present invention.
Fig. 2 is a schematic cross section of a lithium ion
secondary battery according to the embodiment of Fig. 1.
Fig. 3 is a schematic cross section of an embodiment of
conventional lithium ion secondary batteries.
Embodiments for carrying out the present invention will
be illustrated by referring to the accompanying drawing.
Fig. 1 is a schematic cross section of the main part (an
electrode laminate) of a lithium ion secondary battery according
to one embodiment of the present invention. Fig. 2 is a schematic
cross section of a lithium ion secondary battery according to the
embodiment of Fig. 1. In these Figures, numeral 3 indicates a
positive electrode in which active material particles 7a are
adhered to a current collector 6 by means of a binder resin 11.
Numeral 7 is a positive electrode active material layer
comprising active material particles 7a bound by a binder resin
11. Numeral 5 is a negative electrode in which active material
particles 9a are adhered to a current collector 10 by means of
a binder resin 11. Numeral 9 is a negative electrode active
material layer comprising active material particles 9a bound by
a binder resin 11. Numeral 4 indicates a separator that is
interposed between the positive electrode 3 and the negative
electrode 5 and is adhered to the positive and negative active
material layers 7 and 9 with the binder resin 11. Numeral 12
indicates voids formed in the positive and negative active
material layers 7 and 9 and the separator 4, where an electrolytic
solution containing lithium ions is held. Numeral 30 is an
aluminum laminate film which encloses and seals the electrode
laminate shown in Fig. 1 after injection of the electrolytic
solution.
The lithium ion secondary battery having the above-described
structure can be produced by, for embodiment, as
follows.
An active material paste prepared by dispersing positive
electrode active material particles 7a and a binder resin 11 in
a solvent is applied to a positive electrode current collector
6 by roll coating and dried to prepare a positive electrode 3.
A negative electrode 6 is prepared in the same manner. The binder
resin 11 is applied to the both sides of a separator 4 as an adhesive,
and the positive electrode 3 and the negative electrode 6 are each
adhered thereto. The adhesive is dried by hot roll pressing to
prepare an electrode laminate. The whole electrode laminate is
impregnated with an electrolytic solution by dipping and packed
in an aluminum laminate film (resin coated aluminum film) 30. The
film 30 is closed by heat sealing to obtain a lithium ion secondary
battery.
In this embodiment, the adhesive strength between the
separator 4 and each of the positive and negative electrode active
material layers 7 and 9 is not lower than that between the positive
electrode current collector 6 and the positive electrode active
material layer 7 and that between the negative electrode current
collector 10 and the negative electrode active material layer 9.
Further, the coating ratio of the binder resin 11 on the active
material particles 7a and 9a is higher in the side of the separator
4 than in the side of the current collector 6 and 10, respectively.
According to the above-described embodiment, the active
materials and their respective collectors in the electrodes 3 and
5 are adhered together with the binder resin 11 to retain the shape
of the electrodes as in conventional lithium ion secondary
batteries. Since the positive and negative electrodes 3 and 5
(i.e., the positive and negative electrode active material layers
7 and 9) are similarly adhered to the separator 4 with the same
binder resin 11, electrical connections between each of the active
material layers 7 and 9 and the separator 4 can be maintained
without applying an outer force. That is, a hard case for
retaining a battery structure is no more necessary, which makes
it feasible to reduce the weight and thickness of a battery and
to design the shape of a battery freely. Moreover, the separator
4 is adhered to each of the positive and negative active material
layers 7 and 9 with such adhesive strength that is equal to or
above the adhesive strength between the active material and its
current collector in the inside of each electrode, i.e., between
the positive electrode collector 6 and the positive electrode
active material layer 7 and between the negative electrode current
collector 10 and the negative electrode active material layer 9.
As a result, fracture of the electrodes takes place
preferentially over interlaminar delamination between the
separator 4 and the positive and negative electrode active
material layers 7 and 9. For example, when any outer force that
may deform a battery or an internal thermal stress is applied,
it is not the separator but the electrode structure that is broken,
whereby safety can be secured.
In order to further enhance the adhesion between each
electrode and the separator thereby to make the above effect more
outstanding, it is preferable to form a thin binder resin layer
between each electrode and the separator.
In addition, since the binder resin serving as an adhesive
in the positive and negative electrode active material layers 7
and 9 is more abundant in the side (on the surface) of the separator
4. That is, the active material particles 7a and 9a which are
positioned near the separator 4 are coated with the binder resin
11 more than those positioned near the respective collectors 6
and 10. While intercalation and disintercalation of lithium ions
are usually supposed to occur preferentially in the vicinities
of the separator over the inside of the positive and negative
active material layers 7 and 9, the difference in coating ratio
in the thickness direction of the electrode layer relaxes the
difference in speed of intercalation and disintercalation of
lithium ions between the separator 4 side and the inside of the
positive and negative electrode active material layers 7 and 9.
As a result, the active material inside the electrode can be
utilized effectively to improve the charge and discharge
efficiency. It follows that the charge and discharge
characteristics as a battery can be improved.
The active material which can be used in the positive
electrode includes complex oxides of lithium and a transition
metal, such as cobalt, nickel or manganese; chalcogen compounds
containing lithium or complex compounds thereof; and these
complex oxides, Li-containing chalcogen compounds or complex
compounds thereof that contain various dopant elements. While
any substance capable of intercalating and disintercalating
lithium ions, which take the main part of the battery behavior,
can be used as a negative electrode active material, preferred
active materials for use in the negative electrode include
carbonaceous compounds, such as graphitizing carbon, non-graphitizing
carbon, polyacene, and polyacetylene; and aromatic
hydrocarbon compounds having an acene structure, such as pyrene
and perylene.
The active materials are used in a powdered state.
Particles having a particle size of 0.3 to 20 µm can be used. A
preferred particle size is 1 to 5 µm. In the case that the
particle size is too small, the ratio of the surface area of the
active material particles coated with the binder resin becomes
too high to perform lithium ion intercalation and
disintercalation efficiently, resulting in reduction of battery
characteristics. In the case that the particle size is too large,
it is not easy to form the active material paste into a thin film,
the packing density is reduced, and the surface of the electrode
plates formed has large unevenness, resulting in a failure of
sufficient adhesion to the separator.
The binder resin which can be used in binding the active
material and forming into an electrode plate is not particularly
limited as long as it is insoluble in an electrolytic solution
and undergoes or induces no electrochemical reaction in the
electrode laminate. Examples of useful binder resins are homo- or
copolymers of vinylidene fluoride, ethylene fluoride,
acrylonitrile, and ethylene oxide, and ethylene propylenediamine
rubber.
Any metal stable within a battery can be used as a current
collector. Aluminum is preferred for a positive electrode, and
copper is preferred for a negative electrode. The current
collector can be foil, net, expanded metal, etc. Those presenting
a large surface area, such as net and expanded metal, are preferred
from the standpoint of ease of holding an electrolytic solution
after adhesion.
Any separator which is electron-insulating and has
sufficient strength, such as porous film, net, and nonwoven fabric,
can be used. In using a separator made of a fluorine resin, there
are some cases in which the adhesive strength should be increased
by surface treatment, such as plasma treatment. While not
limiting, polyethylene or polypropylene is a preferred material
for the separator for their adhesiveness and safety.
The solvent and the electrolyte which provide an
electrolytic solution serving as an ion conductor can be any of
nonaqueous solvents and any of lithium-containing electrolyte
salts that have been employed in conventional batteries.
Examples of useful solvents include ethers, such as
dimethoxyethane, diethoxyethane, diethyl ether, and dimethyl
ether; esters, such as propylene carbonate, ethylene carbonate,
diethyl carbonate, and dimethyl carbonate; and mixed solvents
consisting of two members selected from the ether solvents or the
ester solvents or mixed solvents consisting of one member selected
from the former group and one member selected from the latter group.
Examples of useful electrolytes are LiPF6, LiAsF6, LiClO4, LiBF4,
LiCF3SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3.
The adhesive resin which can be used for adhering a
current collector and an electrode can be selected from those
insoluble in an electrolytic solution and undergoes or induces
no electrochemical reaction in the electrode laminate similarly
to the adhesive resins used for adhering an electrode and a
separator. Examples of useful adhesive resins are crosslinked
methacrylic or acrylic resins and homo- or copolymers of ethylene,
styrene, vinylidene fluoride, etc.
The present invention will now be illustrated in greater
detail with reference to Embodiments, but it should be understood
that the present invention is not deemed to be limited thereto.
Unless otherwise noted, all the percents and parts are by weight.
Eighty-seven (87) parts of LiCoO2, 8 parts of graphite
powder (KS-6, produced by Lonza Ltd), and, as a binder resin,
5 parts of polyvinylidene fluoride (hereinafter abbreviated as
PVDF) were dispersed in N-methylpyrrolidone (hereinafter
abbreviated as NMP) to prepare a positive electrode active
material paste. The paste was applied to 20 µm thick aluminum
foil as a current collector by using a doctor blade method to a
thickness of about 100 µm to prepare a positive electrode.
Ninety-five parts (95) of Mesophase Microbead Carbon (a
trade name, produced by Osaka Gas Co., Ltd.) and, as a binder resin,
5 parts of PVDF were dispersed in NMP to prepare a negative
electrode active material paste. The paste was applied to 12 µm
thick copper foil as a current collector with a doctor blade to
a thickness of about 100 µm to prepare a negative electrode.
A NMP solution containing 5% by weight of PVDF, which
was used above as an binder resin for adhering active material
particles onto a current collector, was uniformly coated to one
side of a separator (Cellguard #2400, produced by Hoechst Celanese
Plastics Ltd.), and the positive electrode was adhered thereon
uniformly. On the other side of the separator was coated the
same NMP solution of PVDF, and the negative electrode was adhered
thereto. After the electrode laminate thoroughly dried, an
electrolytic solution of lithium hexafluorophosphate
(electrolyte) in a mixed solvent of ethylene carbonate and
1,2-dimethoxyethane was injected into the electrode laminate, and
the impregnated laminate was heat-sealed in an aluminum laminate
film pack to complete a lithium ion secondary battery.
The resulting battery stably retained its shape without
imposing pressure externally, keeping an electrical connection
between electrodes. When, after the assembly, the aluminum
laminate film was removed, and the electrodes were peeled off the
separator, the active material layers were found remaining on the
separator. This proves that the interlaminar adhesive strength
between the separator and the active material layer is greater
than the interlaminar adhesive strength between the current
collector and the active material layer. This seems to be because
the binder resin as an adhesive was present more on the separator
side than on the current collector side. In this manner, because
fracture of the electrode(s) occurs preferentially over
interlaminar delamination between the active material layer(s)
and the separator, the battery can maintain safety.
On evaluating the battery characteristics, the energy
density per unit weight was 120 Wh/kg owing to effective
utilization of the active materials in the inside of the active
material layers. Even after 200 charge and discharge cycles at
a current of C/2, the charge capacity was as high as 75% of the
initial level. This is believed to be because the binder resin
was present more on the separator side than on the current
collector side. In other words, the active material particles
positioned nearer to the separator were coated with the binder
resin to a larger extent than those positioned nearer to the
current collector, so that the difference in speed of
intercalation and disintercalation of lithium ions between the
separator side and the inside of the electrode active material
layers might be reduced by offsetting and, as a result, the active
materials inside the electrodes were made effective use of.
Thus, since a hard battery case is no more necessary, it
is feasible to produce batteries which can have reduced weight
and thickness and an arbitrary shape. Besides, achievement of
an improvement in charge and discharge efficiency enables
production of lithium ion secondary batteries having excellent
charge and discharge characteristics and high safety.
A battery was produced in the same manner as in Embodiment
1, except that the positive and negative electrode active material
layers both had a thickness of about 200 µm. Similarly to
Embodiment 1, the resulting battery stably retained its shape
without imposing pressure externally, keeping an electrical
connection between electrodes. When, after the assembly, the
aluminum laminate film was removed, and the electrodes were
stripped off the separator, the active material layers were found
remaining on the separator, proving that the interlaminar
adhesive strength between the separator and the active material
layer is greater than the interlaminar adhesive strength between
the current collector and the active material layer.
On evaluating the battery characteristics, the energy
density per unit weight was 103 Wh/kg and, even after 200 charge
and discharge cycles at a current of C/2, the charge capacity was
as high as 60% of the initial level. It was thus proved that a
lithium ion secondary battery having reduced weight and thickness,
an arbitrary shape, and excellent charge and discharge
characteristics could be produced similarly to Embodiment 1.
A battery was produced in the same manner as in Embodiment
1, except that a NMP solution containing 12% by weight of PVDF
was used for adhesion of the positive and negative electrode
active material layers to the separator. Similarly to Embodiment
1, the resulting battery stably retained its shape without
imposing pressure externally, keeping an electrical connection
between electrodes. When, after the assembly, the aluminum
laminate film was removed, and the electrodes were peeled off the
separator, the active material layers were found remaining on the
separator, indicating that the interlaminar adhesive strength
between the separator and the active material layer was greater
than the interlaminar adhesive strength between the current
collector and the active material layer. This is considered to
be because a PVDF solution having a high concentration formed a
thin PVDF layer between the separator and the electrode to enhance
the interlaminar adhesive strength, maintaining an electrical
connection stably.
On evaluating the battery characteristics, the energy
density per unit weight was 90 Wh/kg and, even after 200 charge
and discharge cycles at a current of C/2, the charge capacity was
as high as 80% of the initial level. Similarly to Embodiment 1,
it was proved that a lithium ion secondary battery having reduced
weight and thickness, an arbitrary shape, and excellent charge
and discharge characteristics could be produced.
Eighty-seven parts of LiCoO2, 8 parts of graphite powder
(KS-6, produced by Lonza Ltd.), and, as a binder resin, 5 parts
of polystyrene powder were mixed, and appropriate amounts of
toluene and 2-propanol were added thereto to prepare a positive
electrode active material paste. The paste was applied to 20 µm
thick aluminum foil as a current collector by using a doctor blade
method to a thickness of about 100 µm to prepare a positive
electrode.
Ninety-five parts of Mesophase Microbead Carbon (a trade
name, produced by Osaka Gas Co., Ltd.) and, as a binder resin,
5 parts of polystyrene powder were mixed, and appropriate amounts
of toluene and 2-propanol were added thereto to prepare a negative
electrode active material paste. The paste was applied to 12 µm
thick copper foil as a current collector by using a doctor blade
method to a thickness of about 100 µm to prepare a negative
electrode.
A toluene solution containing 5% by weight of
polystyrene was uniformly applied to a side of a nitrocellulose
porous film (pore size: 0.8 µm) as a separator, and the positive
electrode was stuck thereto uniformly. On the other side of the
separator was uniformly applied the same toluene solution of
polystyrene, and the negative electrode was stuck thereto. After
the electrode laminate thoroughly dried, an electrolytic solution
of lithium hexafluorophosphate (electrolyte) in a mixed solvent
of ethylene carbonate and 1,2-dimethoxyethane was injected into
the electrode laminate, and the impregnated laminate was
heat-sealed in an aluminum laminate film pack to complete a
lithium ion secondary battery.
The resulting battery stably retained its shape without
imposing pressure externally, keeping an electrical connection
between electrodes. When, after the assembly, the aluminum
laminate film was removed, and the electrodes were peeled off the
separator, the active material layers were found remaining on the
separator, indicating that the interlaminar adhesive strength
between the separator and the active material layer was greater
than the adhesive strength in the inside of the electrode and the
interlaminar adhesive strength between the current collector and
the active material layer. On evaluating the battery
characteristics, the energy density per unit weight was about
90 Wh/kg, and even after 100 charge and discharge cycles at a
current of C/10, about 60% of the initial charge capacity was
retained. Similarly to Embodiment 1, it was proved that a lithium
ion secondary battery having reduced weight and thickness, an
arbitrary shape, and excellent charge and discharge
characteristics could be produced.
A positive electrode and a negative electrode were
prepared in the same manner as in Embodiment 1. A toluene
solution containing 10% by weight of poly-methyl methacrylate,
the same binder resin as used for adhering active material
particles to the current collector, was used for adhering the
electrodes to the separator. After the adhesion, the electrode
laminate was dried in vacuum while heating at 80°C. The resulting
battery stably retained its shape without imposing pressure
externally, keeping an electrical connection between electrodes.
When, after the assembly, the aluminum laminate film was removed,
and the electrodes were peeled off the separator, the active
material layers were found remaining on the separator, proving
that the interlaminar adhesive strength between the separator and
the active material layer was greater than the interlaminar
adhesive strength between the current collector and the active
material layer. On evaluating the battery characteristics, the
energy density per unit weight was about 103 Wh/kg, and even after
100 charge and discharge cycles at a current of C/2, the charge
capacity was as high as about 80% of the initial level.
Similarly to Embodiment 1, it was proved that a lithium
ion secondary battery having reduced weight and thickness, an
arbitrary shape, and excellent charge and discharge
characteristics could be produced.
Eighty-seven (87) weight parts of LiCoO2, 8 weight parts
of graphite powder (KS-6, produced by Lonza Ltd), and, as a binder
resin, 5 weight parts of PVDF were dispersed in NMP to prepare
a positive electrode active material paste. The paste was applied
to 20 µm thick aluminum foil as a current collector with a doctor
blade to a thickness of about 100 µm to prepare a positive
electrode.
Ninety-five (95) weight parts of Mesophase Microbead
Carbon (a trade name, produced by Osaka Gas Co., Ltd.) and, as
a binder resin, 5 weight parts of PVDF were dispersed in NMP to
prepare a negative electrode active material paste. The paste
was applied to 12 µm thick copper foil as a current collector with
a doctor blade to a thickness of about 100 µm to prepare a negative
electrode.
The aluminum foil coated with the positive electrode
active material and the copper foil coated with the negative
electrode active material were pressed, while wet, onto each side
a separator (Cellguard #2400, produced by Hoechst Celanese
Plastics Ltd.) and dried to prepare an electrode laminate having
the separator as an interlayer. After the electrode laminate
thoroughly dried, an electrolytic solution of lithium
hexafluorophosphate (electrolyte) in a mixed solvent of ethylene
carbonate and 1,2-dimethoxyethane was injected into the electrode
laminate, and the impregnated laminate was heat-sealed in an
aluminum laminate film pack to complete a lithium ion secondary
battery.
The resulting battery stably retained its shape without
imposing pressure externally, keeping an electrical connection
between electrodes. When, after the assembly, the aluminum
laminate film was removed, and the electrodes were peeled off the
separator, the active material was found remaining on the
separator only sparsely. This indicates that the interlaminar
adhesive strength between the separator and the active material
layer is very lower than to that between the current collector
and the active material layer. On evaluating the battery
characteristics, the energy density per unit weight was 70 Wh/kg.
After 200 charge and discharge cycles at a current of C/2, the
charge capacity was as low as 40% of the initial level. That is,
the resulting battery was considerably inferior in battery
characteristics to those produced in the foregoing Embodiments,
which verifies that adhering of positive and negative electrodes
to a separator with an adhesive results in improvements in battery
characteristics. In other words, it can be seen that the
distribution of an adhesive and a binder resin plays an important
role in improving battery characteristics.
It is not necessary required to use a same binder resin
as a binder resin which is used in electrode active material
layer. A different material from that used in electrode active
material layer of can be used as a binder resin to adhere a
separator to a current collector. The first structure of
the lithium ion secondary battery according to the present
invention has a positive electrode comprising a current collector
having adhered thereto positive electrode active material
particles with a binder resin, a negative electrode comprising
a current collector having adhered thereto negative electrode
active material particles with a binder resin, a separator which
is interposed between the positive electrode and the negative
electrode and is adhered to each of positive and negative active
material layers, and a lithium ion-containing electrolytic
solution which is held in voids of the positive and negative
electrode active material layers and the separator, in which the
adhesive strength between the separator and each of the positive
and negative electrode active material layers is not lower than
that between the positive electrode current collector and the
positive electrode active material layer and that between the
negative electrode current collector and the negative electrode
active material layer. According to this structure, a hard case
for retaining a battery structure is no more necessary, which
makes if feasible to reduce the weight and thickness of a battery
and to design the shape of a battery freely. Additionally, since
the charge and discharge efficiency can be improved, there is
provided a lithium ion secondary battery having excellent charge
and discharge characteristics and high safety.
The second structure of the lithium ion secondary battery
according to the present invention has the first structure, in
which each of the positive and negative electrode active material
layers is adhered to the separator with the same binder resin used
for adhering the positive electrode active material particles and
the negative electrode active material particles to the
respective current collectors. According to this structure, the
battery has improved reliability.
The third structure of the lithium ion secondary battery
according to the present invention has the first or second
structure, in which the coating ratio of the binder resin on the
active material particles is higher in the side of the separator
than in the side of the current collector. According to this
structure, the difference in speed of intercalation and
disintercalation of lithium ions between the active material
positioned near to the separator and that positioned near to the
current collector can be relaxed so that the active material
inside the electrode can be utilized effectively to bring about
improved charge and discharge efficiency.
Claims (11)
- A lithium ion secondary battery comprising:a positive electrode having a positive electrode collector and a positive electrode active material layer in which positive active material particles are dispersed and bound with a binder resin to the positive electrode collector;a negative electrode having a negative electrode collector and a negative electrode active material layer in which negative active material particles are dispersed and bound with a binder resin;a separator which is interposed between said positive electrode and said negative electrode and is adhered to each of positive and negative active material layers; anda lithium ion-containing electrolytic solution which is held in voids of said positive and negative electrode active material layers and said separator,
wherein the adhesive strength between said separator and each of said positive and negative electrode active material layers is not lower than that between said positive electrode current collector and said positive electrode active material layer and that between said negative electrode current collector and said negative electrode active material layer. - The lithium ion secondary battery according to claim 1, wherein each of said positive and negative electrode active material layers is adhered to said separator with the same binder resin as used for adhering each of said positive active material particles and said negative active material particles to the respective current collectors.
- The lithium ion secondary battery according to claim 1, wherein each of said positive and negative electrode active material layers is adhered to said separator with a different binder resin from the binder resin used for adhering said positive active material particles and said negative active material particles to the respective current collectors.
- The lithium ion secondary battery according to claim 1, wherein the coating ratio of said binder resin on said active material particles is higher in the side of said separator than in the side of said current collector.
- The lithium ion secondary battery according to claim 1, wherein the density of the active material particles in each of the positive and negative electrode material layers in the side of the separator is lower than that of in the side of each of the positive and negative current collector.
- The lithium ion secondary battery according to claim 1, wherein the density of the binder resin in each of the positive and negative electrode material layers in the side of the separator is higher than that of in the side of each of the positive and negative current collector.
- A lithium ion secondary battery comprising:a positive electrode having a positive electrode collector and a positive electrode active material layer in which particles of a positive active material are dispersed and bound with a binder resin to the positive electrode collector;a negative electrode having a negative electrode collector and a negative electrode active material layer in which particles of a negative active material are dispersed and bound with a binder resin;a separator which is interposed between said positive electrode and said negative electrode and is adhered to each of positive and negative active material layers; anda lithium ion-containing electrolytic solution which is held in voids of said positive and negative electrode active material layers and said separator,
wherein the adhesive strength between said separator and each of said positive and negative electrode active material layers is not lower than that between said particles in each of said positive electrode active material layer and said negative electrode active material layer respectively. - The lithium ion secondary battery according to claim 7, wherein the density of the active material particles in each of the positive and negative electrode material layers in the side of the separator is lower than that of in the side of each of the positive and negative current collector.
- The lithium ion secondary battery according to claim 7, wherein the density of binder resin in each of the positive and negative electrode material layers in the side of the separator is higher than that of in the side of each of the positive and negative current collector.
- The lithium ion secondary battery according to claim 7, wherein the active material particles have a particle size in a range of 0.3 to 20 µm.
- The lithium ion secondary battery according to claim 7, wherein the active material particles have a particle size in a range of 1 to 5 µm.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP34760096 | 1996-12-26 | ||
| JP34760096A JP3223824B2 (en) | 1996-12-26 | 1996-12-26 | Lithium ion secondary battery |
| JP347600/96 | 1996-12-26 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| EP0851521A2 true EP0851521A2 (en) | 1998-07-01 |
| EP0851521A3 EP0851521A3 (en) | 2004-08-25 |
Family
ID=18391321
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| EP97122405A Withdrawn EP0851521A3 (en) | 1996-12-26 | 1997-12-18 | Lithium ion secondary battery |
Country Status (3)
| Country | Link |
|---|---|
| US (1) | US6136471A (en) |
| EP (1) | EP0851521A3 (en) |
| JP (1) | JP3223824B2 (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0859417A2 (en) * | 1997-02-12 | 1998-08-19 | Mitsubishi Denki Kabushiki Kaisha | Adhesive for battery, battery using the same and method of fabricating a battery using the same |
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Cited By (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0859417A2 (en) * | 1997-02-12 | 1998-08-19 | Mitsubishi Denki Kabushiki Kaisha | Adhesive for battery, battery using the same and method of fabricating a battery using the same |
| EP0859417A3 (en) * | 1997-02-12 | 2003-05-02 | Mitsubishi Denki Kabushiki Kaisha | Adhesive for battery, battery using the same and method of fabricating a battery using the same |
| US6660433B2 (en) | 1999-12-14 | 2003-12-09 | Sanyo Electric Co., Ltd. | Lithium secondary battery and battery device comprising same |
| GB2357896A (en) * | 1999-12-14 | 2001-07-04 | Sanyo Electric Co | Lithium secondary battery and battery device comprising same |
| GB2357896B (en) * | 1999-12-14 | 2004-03-17 | Sanyo Electric Co | Lithium secondary battery and battery device comprising same |
| US7223495B2 (en) | 2000-04-04 | 2007-05-29 | Sony Corporation | Non-aqueous electrolyte secondary battery |
| EP1672730A3 (en) * | 2000-04-04 | 2007-04-11 | Sony Corporation | Non-aqueous electrolyte secondary battery |
| US7563539B2 (en) | 2000-04-04 | 2009-07-21 | Sony Corporation | Non-aqueous electrolyte secondary battery |
| EP2264813A1 (en) * | 2000-04-04 | 2010-12-22 | Sony Corporation | Non-aqueous electrolyte secondary battery |
| WO2002101865A2 (en) * | 2001-04-03 | 2002-12-19 | Le Carbone Lorraine | Electrochemical cells and method of making the same |
| WO2002101865A3 (en) * | 2001-04-03 | 2004-04-08 | Lorraine Carbone | Electrochemical cells and method of making the same |
| WO2002082571A1 (en) * | 2001-04-03 | 2002-10-17 | Le Carbone Lorraine | Electrochemical cells and method of making the same |
| CN108365164A (en) * | 2017-01-27 | 2018-08-03 | 丰田自动车株式会社 | The manufacturing method of battery |
| CN108365164B (en) * | 2017-01-27 | 2020-11-17 | 丰田自动车株式会社 | Method for manufacturing battery |
Also Published As
| Publication number | Publication date |
|---|---|
| US6136471A (en) | 2000-10-24 |
| JP3223824B2 (en) | 2001-10-29 |
| JPH10233232A (en) | 1998-09-02 |
| EP0851521A3 (en) | 2004-08-25 |
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